Conventional composite structural joints (co-cured, bonded or bolted) are severely limited in out of plane load capacity (as generated by fuel pressure loads in a wing box or kick loads at structural discontinuities). Joint out of plane loads cause high peel stresses and interlaminar stresses in conventional 2-D laminated composite joints as shown in FIGS. 1A-1E.
Typical composite resins have good tension and shear strengths, but very low peel strength.
Significant composite joint improvements were developed as far back as 1974 by using 3-D hand woven textile joint inserts in co-cured wing to span joints. However, due to high cost of the hand woven textiles, this technology was not used until the early 1990s when the F-2 program automated weaving of the deltoid insert shown in FIG. 2.
This 3-D woven insert is primarily used as a radius filler on the lower wing skin to spar co-cured joints. While strength is increased, this application is still limited by expensive tooling and processing required for co-cure fabrication of the F-2 composite wing. Also, the joint is still prone to delaminating up the middle.
The Beech Starship utilized another from of 3-D joint as shown in FIG. 3. This joint utilized sandwich materials, therefore co-curing the entire structure would have been very difficult. Beech opted to precure the detail parts and secondarily bond them together. While this approach worked, it was still fairly load limited and had disadvantages common to secondary bonding (fitup of pre-cured piece to piece). This particular design was also limited to sandwich structures.
The NASA ACT (Advanced Composites Technology) program worked on entire structures that were 3-D woven, knitted, braided or stitched together. While these designs may have benefits in damage tolerance and joint strengths, there are severe limitations. Knitted, braided, stitched and 3-D woven structures typically have out of plane properties that are superior to conventional 2-D structures (made out of fabric and tape), however their in plane properties are generally much lower. This leads to weight penalties when the 3-D materials are used for wing skins, spar or bulkhead webs, fuselage skins, et cetera, that typically have high in plane loads. Also, complex geometry limits use of totally woven or stitched together structures due to machine and processing limitations.
The present invention generally uses conventional composite tape, fabric and/or metal details for structural skins, spar and bulkhead webs, fittings et cetera. Conventional laminates are used where high in plane properties are desired. Many different material combinations are possible such as RTM details, thermoplastic details, fiberglass, BMI, etc. The most cost effective process of fabricating the details can be used, in example, a tape laid, platten press cured, waterjet trimmed spar web. The finished details are located with uncured, resin infused 3-D woven connectors (preforms) and adhesive in between the parts in a simple assembly jig or with self locating tooling features (tooling tabs or pins, etc.) Simple compliant overpresses are then placed over the weaves. The assembly is then vacuum bagged and cured, typically with heat and/or pressure, or E-beam processed to avoid thermal effects. It is also possible to assemble structures with room temperature cure systems (wet layup).
The use of these advanced 3-D woven connectors combined with the co-bond process produces low cost, robust, composite structural joints not obtainable with other prior art. Simple, inexpensive, compliant overpresses can be used since the uncured 3-D textile connector forms against the cured detail parts during processing. This method avoids the precision tools required for co-cure (where all the parts are uncured) or the precise fit up required with secondary bonding (where multiple cured parts are brought together with a thin layer of adhesive in between).
Fabrication of the 3-D woven preforms is conducted on fully automated looms which allows cost effective, high quality, repeatable connectors. Once fabricated into a structure, the 3-D woven connector behaves similar to a fitting in between the detail parts transferring load in shear and tension, not peel.